These studies try to identify critical target genes and alterations in these genes which may be important in chemical carcinogenesis. Certain human tumors and chemically induced tumors in animals have a high frequency of proto-oncogene activation, particularly by point mutation of ras genes, and there is evidence for chemical specificity in the pattern of mutations detected. The specific types of oncogene- activating mutations induced by a chemical carcinogen often agree with what is expected based on the DNA adducts formed by that agent. Even for "non-genotoxic" carcinogens, the patterns of ras gene mutations in tumors can give clues about the mechanism of chemically induced tumorigenesis. We are continuing studies on the role of the K-ras oncogene in susceptibility to pulmonary tumorigenesis in inbred mouse strains. Activation of K-ras by point mutations in chemically induced lung tumors for the resistant C57BL/6 mouse is being related to the persistence of DNA adducts in lungs following treatment with methylating nitrosamines. Mutations are identified by single stranded conformation polymorphism, slot blot oligonucleotide hybridization, restriction fragment length polymorphism and direct sequencing following PCR amplification of ras gene fragments in DNA isolated from the tumors. The nude mouse tumorigenicity assay is also employed to try to detect other transforming genes in tumors lacking activated ras genes. Furthermore, analyses of losses of heterozygosity and microsatellite repeats in spontaneous and chemically induced lung tumors from hybrids between resistant mice are being performed in order to try to identify other susceptibility genes. Also, human lung tumors from certain exposure groups, such as uranium miners, are being analyzed for K-ras and p53 mutations in order to understand mechanisms of carcinogenesis by these exposures. In other studies, activation of ras genes by oxazepam, a widely prescribed tranquilizer, and polybrominated biphenyl (Firemaster), is being investigated in order to help understand potential carcinogenic risk of these compounds to humans.